If the incident light frequency increases (above threshold) while intensity remains constant, what happens to the kinetic energy of emitted electrons?

• A. The kinetic energy decreases
• B. The kinetic energy remains the same
• C. The kinetic energy increases
• D. No electrons are emitted

Answer: (C)

In the photoelectric effect, each photon delivers energy hf. To eject an electron from the surface, at least the work function φ is needed; when hf > φ, the extra energy becomes the kinetic energy of the emitted electron: KE = hf − φ. So, if the incident light frequency increases above the threshold, the energy available to each emitted electron rises, and the kinetic energy increases correspondingly. Keeping intensity constant changes how many photons (and thus how many electrons) are emitted, but not the energy per emitted electron. Therefore the kinetic energy increases with frequency above threshold.